A Festae pair would be fun in a 6ft. Jaguar pair or Red tiger motaguense pair. I personally love red devils and midas... really any of the amphilophus for wet pets. Super interactive and responsive at all sizes.
If you want an Australia native tank with an aggressive fish get mangrove jack! They're mean as hell
Yeah jacks are freshwater in nature they migrate to salt when they're older but that doesn't matter in the aquarium. Na Rtms aren't here from what I'm aware of. As for jags you can vent which I have no idea on how to do but there are tons of videos on YouTube otherwise you can visually sex them but again I have no idea.
They're around $100 + some cheaper some more, generally a bad idea to keep other fish with them, they're similar to dovii but Imo worse
If you want something less aggressive but an Australian native you could go with a coal grunted or a black bream. Something else really cool and aggressive are jardini and leichardti. Lei aren't as aggressive from what I've heard but grow bigger. A good setup for a six footer for a few years (before the jar or lei grow to big) I reckon would be a Barra, coal grunter or black bream, a toga (lei or jar) and a eel tailed cat. Almost forgot Murray cod, BIG and predatory awesome fish. You could keep one with a toga and a Barra I reckon all three of which are really predatory.
